A three phase brushless DC (BLDC) motor is shown in FIG. 1. As can be seen, the motor (shown generally at 100) comprises a rotor 110 made up of one or more permanent magnets and a stator 120 which surrounds the rotor 110 and has first, second and third independently energisable electromagnets 130, 140, 150 equally angularly spaced around the periphery of the stator 120. Each of the electromagnets 130, 140, 150 has a terminal to which an electric current can be applied to energise the electromagnet.
The motor 100 is driven using a three phase electronic bridge. Current is applied to pairs of the electromagnets 130, 140, 150 via their respective terminals in a predetermined sequence, to generate a moving electromagnetic field within the motor which causes the rotor 110 to rotate.
A three phase BLDC motor has six rotational zones where current should be optimally applied to a pair of terminals. These are known as commutations, as the bridge ‘commutes’ the drive at each boundary. Each commutation step covers 60 degrees of rotation of the rotor.
In many sensorless BLDC motor implementations (i.e. implementations in which there are no sensors associated with the electromagnets for determining the position of the rotor), the terminal which is not being used to energise an electromagnet (known as the ‘floating’ terminal) can be used for determining the position of the motor, by measuring a back electromotive force (EMF) induced in the electromagnet due to the rotation of the rotor. It is well known that the voltage at the floating terminal is exactly equal to half of the voltage across the other two terminals at the mid-point between commutation boundaries (i.e. at 30 degrees for a three phase BLDC motor). This mid-point is known as the ‘zero-crossing’ point. A variety of time delay means are used to ensure that the commutation occurs 30 degrees after the zero-crossing point, to ensure efficient commutation.
In an electrically noisy environment it is necessary to filter the floating terminal voltage to eliminate environmental noise that may affect the zero-crossing detection. However, such filters delay the point in time at which the zero-crossing is observed. This delay is more significant at higher speeds and, unless compensated, results in reduced motor efficiency, as commutation occurs at sub-optimal times.
Accordingly, a need exists for a filter that reduces or eliminates high levels of environmental electrical noise in a brushless DC motor without unduly affecting zero-crossing detection and thus commutation accuracy.